The present invention relates generally to a process for manufacturing an explosive propellant. More particularly the invention relates to a coating step in the process for manufacturing explosive propellants using a less energetic material as a coating agent.
To improve the survivability of current gun propellant-based munitions, insensitivity to external factors must be taken into account when designing ordnance. By reducing the vulnerability of propellants to indirect detonation or ignition during battle, production, stockpiling, and transportation, better protection will be afforded to personnel and equipment.
The range of threats to gun propellant varies with the different systems in which it is placed. These threats include shaped-charge jets, kinetic energy penetrators, and hot spall. Therefore, materials must be able to withstand a large degree of impact, shock wave energy, and/or heat depending on the threat or threats. Moreover, friction and electrostatic discharge vulnerability should be low to ensure safe handling and manufacture. Testing, as documented in MIL-STD-2105-A is performed to determine material vulnerability.
The sensitivity requirements of the propellant depend upon the particular battlefield threat to the ordnance into which the propellant will be incorporated. The most common threats are contact with excessive heat, impact, and shock. Particular concern in any new approach to propellants should be addressed.
Propellants are mixtures containing different highly reactive materials; these materials, give the propellant desirable gas-generating properties. However, the sensitivity of the propellant will primarily depend upon the individual sensitivity of its respective ingredients. Designers of propellants find themselves having to balance the need for stability with the need for proper burning rates and energy. Since ballistic performance is a priority in selecting the appropriate propellant, the use of inert materials is very limited, particularly in propellants possessing resistance to accidental ignition, known as LOVA (low vulnerability ammunition) type compositions. Any potential trade-off between sensitivity and performance has led engineers to seek new energetic materials that satisfy certain sensitivity as well as energetic requirements.
Two approaches to enhance insensitivity of propellants have been to reduce the particle size of the filler material and to coat the filler material. Studies have shown that coated fine explosive particles and especially coated fine explosive materials in a composite propellant cause a less violent explosion reaction. This can be attributed to two factors. First, more surface area of the filler is exposed to the binder/plasticizer matrix by using fine particles. Second, hot spots and shear band friction formed in the micro structure of propellant grains with sufficiently small particles may exhibit reduced friction to the point where the temperatures are inadequate for ignition.
Accordingly, one object of the present invention is to provide a gun propellant-based munitions with improved survivability.
Another object of this invention is to reduce the vulnerability of propellants to indirect detonation or ignition during battle, production, stockpiling, and transportation.
Still another object of this invention is to resolve the potential trade-off between sensitivity and performance to satisfy certain sensitivity as well as energetic requirements.
Yet another object of this invention is provide a method in which it would be possible to employ fine explosive particles in a composite propellant.
Other objects will appear hereinafter.
It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, an improved main energetic ingredient filler has now been discovered that has substantially improved survivability when subjected to external factors.
The filler is extremely useful for propellant-based munitions. The filler comprises filler particles having a fine particle size of less than about 10 micrometers diameter and a thin coating of graphite on the filler particles in an amount such that the weight of the graphite is less than about two percent of the weight of the filler particles.
The filler of this invention may be formed into a propellant including a binder and a plasticizer. The method of making the filler of this invention includes the steps of grinding the filler to a fine particle size and coating graphite on the particles in an amount such that the weight of graphite is less than about two percent of the weight of the filler particles. The filler is then used as normal in the formulation of the propellant or other end use for which the filler is intended to be used.
The preferred fillers ale Hexanitrohexaazaisowurtzitane or CL-20, and 1,3,3-Trinitroazetidine on TNAZ. Also preferred are mixtures thereof. Currently used fillers such as RDX (Cyclotrimethylene Trinitramine), NQ (nitroguanidine) and HMX (Cyclotetramethylene Tetranitramine) are also suitable for the present invention. Most preferred is a filler of CL-20 ground to an average particle diameter of about five to ten micrometers.
Preferred filler particles have a fine particle size of less than about ten micrometers diameter, and most preferred are fillers with a particle size having an average particle diameter ranging from about two to about eight micrometers.
The preferred amount of graphite comprises about one percent by weight of the filler particles. When the amount of graphite is less than about 0.1% of the filler weight, the benefits are not as clearly demonstrable by some of the tests. If the graphite exceeds about 5%, based on the weight of the filler, energetic performance suffers. It is necessary to select the proper amount of graphite to balance the trade-off between sensitivity and performance to satisfy both certain sensitivity as well as energetic requirements.